1. Field of the Invention
The present invention is generally directed to a method for forming deposits of a metal species on a support for use as a catalyst. These catalyst are particularly well suited for use as an electrocatalyst in fuel cells.
2. Description of Related Technology
A fuel cell is an electrochemical device for directly converting the chemical energy generated from an oxidation-reduction reaction of a fuel such as hydrogen or hydrocarbon-based fuels and an oxidizer such as oxygen gas (e.g., in air) supplied thereto into a low-voltage direct current. For the oxidation and reduction reactions in a fuel cell to proceed at useful rates, especially at operating temperatures below about 300° C., electrocatalyst materials are typically supplied at the electrodes. Initially, fuel cells used electrocatalysts made of a single metal, usually platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), silver (Ag) or gold (Au) because they are able to withstand the corrosive environment—platinum being the most efficient and stable single-metal electrocatalyst for fuel cells operating below about 300° C. Although platinum is the most efficient and stable single-metal electrocatalyst for fuel cells, it is costly and an increase in electrocatalyst activity over platinum is generally considered to be necessary for wide scale commercialization of fuel cell technology. An improvement in catalysts may take many forms such as increased activity, increase corrosion resistance, increased poison tolerance, and/or decreased costs. For example, increased tolerance to CO has been reported by alloying platinum and ruthenium at a 50:50 atomic ratio (see, D. Chu and S. Gillman, J. Electrochem. Soc. 1996, 143, 1685). Ideally, a reduction in cost will accompany an improvement in one or more of the preceding performance characteristics.
Fuel cell electrocatalysts were first used in fuel cells in metallic powder form. However, techniques have been developed to disperse these metals over the surface of electrically conductive supports (e.g., carbon black) to increase the surface area of the electrocatalyst which in turn increased the number of reactive sites leading to improved efficiency of the cell.